FIG. 1 shows diagrammatically a flat screen 1 with microtips 6 recognized as comprising a first substrate 2 on which is arranged a cathode 4 fitted with microtips 6 controlled electrically by a Vcathode voltage (earthed in the example shown in FIG. 1) and a grid 8 controlled electrically by a Vgrid voltage and provided with holes 10 in the positions of microtips 6. The first substrate 2 can be in glass or silicon or any other appropriate material. The flat screen 1 also comprises a second substrate 12 facing the first substrate 2 and on which is arranged an anode 14 constituting the emissive surface of the screen 1. Electrical conductors 15, transparent or not, are arranged on the internal surface of the anode 14. Phospor materials 17 are deposited on these electrical conductors 15.
In this device, the cathode 4 constitutes the source of electrons and the anode 14 collects these electrons. The emission of electrons by the microtips 6 is controlled by the potential difference (PD) applied between the extraction grid 8 and the cathode 4.
In practice, a matrix cathode is made having n grid conductor lines and p cathode conductor columns controlling the microtips. The electrical signals applied to these n lines and p columns then allow the image display of (n*p)points.
The collection of electrons from the microtips 6 is made by accelerating the latter using the “high voltage” applied on the anode conductors 15. Luminous emission is provided by the phosphor materials 17 under the impact of electrons received.
To make the screen 1, the first and second substrates 2 and 12 are sealed and a vacuum is made in the enclosed space thus obtained. Numerous technological variants of anodes are available. As an example, one can mention:                single wire or multi-wire anodes for monochrome or three colour screens described in the publication from the CEA in the document “Recent development on microtips display at LETI”, R. MEYER. LETI/CEA.        reflective anodes for transparent cathode structure described in the patent N° FR 2 633 763 A of the CEA.        
In all these variants one always finds the need to apply a high voltage to the anode which is generally a pulsatory current. The commutation of this high voltage is necessary, for example, for the generation of temporal colour subframes and for regeneration of the screen.
The generation of temporal colour subframes is a method for controlling the colour based on a sequential addressing of anode tracks carrying red, green and blue phosphors. This sequential addressing is carried out at a sufficiently high frequency so that the base colours, generated in each of the subframes of a pictorial time, “blend together” before the eye of the onlooker.
FIG. 2 represents a flat screen 1 of the prior art in which the anode 14 is monitored by a push-pull circuit 19, arranged outside the screen 1, and using high voltage switching transistors 18, 20. This circuit is connected to the emissive anode 12 by a conductor 22. On this diagram, the monitoring electronic circuits of the lines and columns of the cathode 2 are not shown as they do not concern the invention. The control circuit 19 of the transistors 18, 20 comprises a source of voltage 24 supplying a voltage Vdd and an optoelectronic coupler 26. The logical control signals, and therefore low voltage of this circuit, are symbolized by inputs v1 and v2.
To carry out a regeneration of the screen, the control process consists in scheduling regeneration phases during which a part of the conductors of the anode at least is at low potential and the microtips of the cathode are biased in an emission status. This control mode eliminates certain problems of colour shift.
FIG. 3 is a timing diagram giving an example of signal to be applied to the anode to generate temporal colour subframes.
FIG. 4 is a timing diagram representing an example of signal to be applied to the anode to make the regeneration of the screen 1.
FIG. 5 is a timing diagram representing the logical signals v1 and v2 to obtain the anode control voltage of FIG. 4.
In all cases, it is essential to switch the anode voltage using a high voltage commutation circuit.
Several solutions are used for this purpose among which the control assemblies with capacitive connection, load pump assemblies, etc. These variations of monitoring mode do not however change the principle of the “push-pull” circuit of N-channel Field effect transistors (FET). There is also a variant of this push-pull that uses two complementary transistors—one N-channel FET and one P-channel FET. This circuit is above all used in low voltage, the P-channel FETs having lower voltage resistance than the N-channel FET.
Whatever the method of anode control used, technological evolution of the electron emission screens leads to, in order to increase their brilliance, increasing the anode voltages up to kilovolts, or even dozens of kilovolts. The switching of anode voltages located in these fields then requires the use of rare and expensive special transistors in complicated and bulky assemblies. As an illustration, to switch voltages of 6 kilovolts, a circuit must be made using layouts with several HV transistors of the 1.6 kilovolt type. Apart from the size of these assemblies, they require taking considerable precautions when laying out the circuit considering the voltages handled on the outside of the screen.
The aim of the invention is to overcome the disadvantages mentioned above.
Another aim of the invention is to simplify the design of control circuits of flat screens and to reduce their size.